This invention relates to improvements in or relating to an intake manifold for a multicylinder internal combustion engine capable of optimizing vaporization and distribution of an incoming fuel charge.
In a multicylinder internal combustion engine of the spark ignition type using a fuel injection device or a carburetor, fuel supplied to a suction conduit of the carburetor or a primary suction conduit, for example, is distributed at a riser portion of the intake manifold to flow in streams to branch intake conduits each connected to one of the cylinders. In the event of fuel distribution not being effected equally, fuel-air mixtures introduced into some cylinders will become too enriched while those introduced into the rest of them will become too lean, so that smooth operation of the engine will be interfered with. To effect equal distribution of the fuel requires thorough vaporization of the fuel before introduction into the branch intake conduits. As a means for attaining this end, an extension tube is known which is mounted in the intake manifold and is connected to the suction conduit of the carburetor in such a manner as to open at its end into air streams. Also known in the art is to heat riser the riser portion of the intake manifold by the cooling fluid of the engine. However, the use of the cooling fluid of the internal combustion engine for heating the riser portion of the intake manifold is unable to achieve satisfactory results due to the fact that the temperature of the engine cooling fluid takes time to rise following engines startup. To eliminate this disadvantage, proposals have been made to use a quick heating type of device including a ceramic heater having positive-temperature-coefficient (hereinafter PTC) thermistors.
The extension tube referred to hereinabove is still in the experimental stage and not put to practical use yet. Thus no extension tubes of easy fabrication and ready mounting are known which lend themselves to mass production. One of the problems yet to be solved with regard to extension tubes of the prior art is that although the use of an extension tube contributes to obtaining equal distribution of the fuel, the resistance offered by the extension tube to the flow of fuel-air mixtures causes a reduction in suction efficiency. Also, the use in the intake manifold riser of a quick heating type device including a ceramic heater having PTC thermistors is itself very effective in obtaining vaporization of the fuel. However, there has been no report in the literature on the synergistic effect of the use of this type of heater in optimum combination with an extension tube.
As viewed in a bottom plan view, an intake manifold for a multicylinder internal combustion engine has a construction wherein extensions of the center lines of its branch intake conduits are concentrated in a small part of the riser portion. Thus it is a conclusive factor concerned in obtaining improvements in the distribution of the fuel that the fuel be concentrated in this part and quickly and thoroughly vaporized therein.